There was ~172,000,000 acres of fire annually prior to European colonization
Prior to European colonization most of the fires were located in the central and eastern regions, and were most common in the grassland ecosystems, where replacement fires could have happened every 1-3 years in those systems. The pine, oak and hickory forests of the east (especially the longleaf pine ecosystems of the southeast) would typically have surface fires every 5-10 years, with replacement fires being rare.
Out west where we see the most fires to day the forested ecosystems would have had fairly frequent fires. Those states had less fire overall due to the heterogeniety of ecosystems, that is most of those states have a mix of fire-dependant and fire-independant ecosystems.
Please note that ranges shift based on the fire type selected.